To stack the mail, most stackers use a constantly running roller and a stacking deck. Mail that is to be stacked enters the nip formed by the previous mailpiece (or a spring-loaded paddle) and the roller. When a mailpiece enters the nip, the roller drives it into the stacking deck. until the leading edge of the mailpiece hits a registration wall on the stacking deck. This roller is positioned a certain distance from the vertical wall to allow a portion of the mailpiece to stick out past the roller.
A typical prior art mixed mail stacker is shown in FIG. 1. As shown, the mixed mail stacker 110 includes a stacking deck 120 and a spring-loaded paddle 124 to support the stacked mailpieces 90. In addition, a registration wall 122 is used to align the leading edge of the stacked mailpieces. Incoming mailpieces are usually moved separately toward the mixed mail stacker 110 for stacking. In FIG. 1, reference numeral 100 denotes an incoming mailpiece which is moved by a moving device 128 toward an inlet nip 130 which is formed by an inlet roller 132 and an idler 134. After a mailpiece is ingested by the inlet nip 130, it is moved by the inlet roller 132 and the idler 134 until the trailing edge 97 has passed the inlet nip 130. As shown, as the leading edge 95 of a mailpiece 98 has passed the inlet nip 130, the mailpiece 98 continues to be moved toward the registration wall 122 by a nip 135 formed by a stacking roller 136 and the last mailpiece 108 of the stacked mailpieces 102. But for the first mailpiece 92 to be stacked, it is driven by a nip 135 formed by the stacking roller 136 and the paddle 124 after mailpiece 92 has passed the inlet nip 130. The inlet roller 132 and the stacking roller 136 are driven by a belt 138. After the mailpiece 98 is engaged in the stacking roller 136, its leading edge 95 is bent by the mailpieces already in the stack as illustrated. Therefore, the trailing edge 97 has a tendency to move away from the belt 138, leaving a gap 139 therebetween. When the next mailpiece 100 is ingested by the roller nip 130, the leading edge of the mailpiece 100 will enter the gap 139 between the mailpiece 98 and the belt 138.
The major disadvantage of this type of mixed mail stacker is that it is limited to handling, a very narrow range of mailpiece sizes without adjustment. In order that a mailpiece can be stacked, it must be long enough to have its leading edge engaged in the driven roller 136 before its trailing edge leaves the inlet nip 130. In this case, positive control of the mailpiece is always maintained. If the mailpiece is too short, it may not be engaged in the stacking roller 136 even after its trailing edge has left the roller nip 130. Thus, positive control is lost on that mailpiece. Consequently, the trailing edge of that short mailpiece may not separate from the belt 138. Not only is the mailpiece improperly stacked because the leading edge may not finally reach the registration wall, but its trailing edge may collide with the leading edge of the following mailpiece, possibly causing a mechanical jam or damage to the mailpieces. If, however, the mailpiece is too long, it cannot be stacked at all because its trailing edge cannot become disengaged from the roller nip 130. Thus, most stackers of this type can only handle a maximum, length variation of 2-3 inches.
It is advantageous and desirable to provide a stacker which can be used to stack mixed mailpieces having different sizes and thickness, such as ranging from a postcard (4".times.5.5".times.0.007" or 102 mm.times.140 mm.times.0.178 mm) to a large flat (12".times.15".times.0.5" or 305 mm.times.381 mm.times.12.7 mm), for example, without the need for adjusting the registration wall of the mixed mail stacking apparatus to fit the size of the mailpieces.